Orthodontics is a specialized field of dentistry concerned with the therapeutic movement of malpositioned teeth into proper positions. Orthodontic treatment can provide many benefits to a patient, including improved bite function and speech, enhanced facial aesthetics, and easier maintenance of dental hygiene. To move teeth, the orthodontic practitioner generally prescribes the use of corrective appliances that mechanically engage to the patient's teeth and apply gentle continuous forces that gradually move the teeth into orthodontically correct positions. Treatment often lasts between two and three years, depending on the complexity of the case.
A common mode of treatment uses tiny slotted appliances called orthodontic brackets, which are adhesively attached to the surfaces of the teeth. A resilient arch-shaped wire (“archwire”) is generally tied, or “ligated,” into the slot of each bracket. The ends of the archwire are generally captured in tube-shaped appliances called molar tubes, which are affixed to the patient's molar teeth. While the archwire is initially distorted when ligated to the brackets, it gradually returns toward its original shape, functioning as a track that guides movement of teeth toward desired positions. The brackets, tubes, and archwire are collectively known as “braces.”
Traditional brackets are ligated to the archwire with the help of one or more pairs of opposing tiewings, which are cleat-like projections on the bracket body. The archwire is placed in the archwire slot and generally a tiny elastomeric “O”-ring ligature, or alternatively a metal ligature wire, is fastened over the archwire and beneath the undercut portions of tiewings located on opposite sides of the archwire slot. In this manner, the ligature secures the archwire in the archwire slot of each bracket and obtains a mechanical coupling between these bodies.
The use of ligatures, however, can present numerous drawbacks. When initially installed, for example, elastomeric ligatures tend to secure the archwire very tightly, resulting in relatively high resistance to sliding. In certain stages of treatment, for example when teeth are being leveled and aligned, this can undesirably slow teeth movement. These ligatures also have a tendency to lose elasticity, causing the sliding mechanics of the archwire to change over time. Moreover, the process of stretching each “O”-ring over the archwire and under the tiewings of each bracket can be cumbersome and time-consuming for the orthodontic practitioner. Finally, ligatures can trap food or plaque in areas beneath the bracket tiewings, making cleaning areas around the brackets more difficult. The use of ligature wire results in many of the same problems as elastomeric ligatures above.
Self-ligating brackets can alleviate many of the above problems. These appliances typically use a permanently installed movable component, such as a clip, spring member, door, shutter, bail, or other ligation mechanism. This mechanism encloses the archwire in the archwire slot, obviating a separate ligature. In many cases, self-ligating brackets still include tiewings on the bracket body in the event that the practitioner desires to further secure the archwire in the slot using a separate ligature. In some cases, ligatures may be so used when the practitioner desires to fully seat the archwire into the slot or increase friction between archwire and bracket to further a particular treatment goal.
The ligation mechanism of self-ligating brackets offers many potential advantages. For example, these appliances can decrease friction between the archwire and the bracket compared with brackets ligated with elastomeric ligatures, potentially providing faster movement of teeth in early stages of treatment. Depending on the mechanism, these appliances can also simplify the installation and removal of an archwire, thereby reducing chair time for the treating professional. Finally, self-ligating brackets can also avoid the hygiene issues of conventional brackets above.
Some self-ligating brackets provide “active ligation” by imparting forces to seat a ligated archwire into its archwire slot, enabling the prescription of the appliance to be fully expressed during treatment. The intended benefit of such a configuration is that, in general terms, the range of engagement (e.g., labial-lingual engagement) between the bracket and wire is increased, thus resulting in improved control and alignment of teeth in treatment. By contrast, “passive” self-ligating brackets have a slot depth sufficiently large such that the ligating door does not exert a continuous force seating the archwire into the slot. Still other brackets are engineered to be either active or passive, depending on the size and configuration of the archwire.